Apparently, importing ~/.config/kglobalshortcutsrc is enough for the shortcut to show up in the System Settings, but not enough to load the shortcut. Disabling, applying and re-enabling and applying again also doesn’t help, and neither does re-assigning to the same shortcut, which resulted in this lovely error dialog.
The Alt key is Alt. Why would it need another label?
There’s quite a long convoluted history to key names. In the early days there were META1…META4… which were typically assigned to SHIFT, CONTROL, ALTERNATE, COMMAND, OPTION, [OS NAME], and others.